1. Field of the Invention
The invention relates to a contact-free operating electronic switchgear, having an oscillator capable of being externally affected, a switching amplifier with at least two operating units disposed downstream of the oscillator, an electronic switch, e.g. a transistor, a thyristor or a triac, which can be controlled by the oscillator via the switching amplifier, and a supply circuit for the provision of the supply voltage for the oscillator and for the switching amplifier.
2. The Prior Art
Electronic switchgear of the type basically addressed herein is contact-free and has been increasingly used for approximately twenty years in place of electrical, mechanically activated switchgear with contacts, in particular in connection with electrical or electronic measurement, open or closed loop control circuits. This is true in particular for so-called proximity switches, i.e. for electronic switchgear which operates contact-free. An indication is given by means of such proximity switches whether an activating element, to which the corresponding proximity switch is sensitive, has come sufficiently close to the proximity switch. If an activating element, to which the corresponding proximity switch is sensitive, has come sufficiently close to the proximity switch, the electronic switch is reversed. In a switchgear acting as a closing element the non-conducting electronic switch now becomes conducting, while in a switchgear acting as an opener the conducting electronic switch now inhibits. (By means of a switch-gear of the type under discussion it is also possible to indicate whether a physical quantity of an actuation medium to which the switchgear is sensitive has reached a corresponding value.) The oscillator which be externally affected as an essential component of electronic switchgear.
In regard to the mode of affecting the oscillator, differentiation between inductive and capacitive control is made. In connection with an electronic, contact-free switchgear with inductive control of the oscillator, it is true for the oscillator, as long as a metallic part has not yet reached a preselected distance, that K.times.V=1, with K=feedback factor and V=amplification factor of the oscillator, i.e. the oscillator oscillates. When the respective metallic part has reached the pre-selected distance, the increasing damping of the oscillator leads to a reduction of the amplification factor V, so that K.times.V&lt;1, i.e. the oscillator ceases to oscillate.
In connection with an electronic, contact-free switchgear with capacitive control of the oscillator, it is true for the oscillator, as long as an actuating body has not sufficiently increased the capacitance between an actuating electrode and a backplate electrode, i.e. has not reached a pre-selected distance, that K.times.V&lt;1, i.e. the oscillator does not oscillate. When the actuating body has reached the pre-selected distance, the increasing capacitance between the actuating electrode and the backplate electrode leads to an increase in the feedback factor K, so that K.times.V=1, i.e. the oscillator begins to oscillate. In both embodiments, inductive proximity switch and capacitive proximity switch, the electronic switch, e.g. a transistor, a thyristor or a triac, is controlled depending on the different sides of the oscillator.
In the beginning electronic contact-free switchgear was subject to a number of problems, in comparison with electrical mechanically actuated switchgear, namely among others "Provision of a Supply Voltage for the Oscillator and the Switching Amplifier", "Design of the Oscillator", "Resistance to Short Circuits" and "Activating Pulse Prevention". Addressing these problems and their solutions (as well as other problems and their solutions relating to electronic contact-free switchgear) are, for example, German Non-examined or Examined Published Applications or Patent Nos. 19 51 137, 19 66 178, 19 66 213, 20 36 840, 21 27 956, 22 03 039, 22 03 040, 22 03 906, 23 30 233, 23 31 732, 23 56 490, 26 13 423, 26 16 265, 26 16 773, 26 28 427, 27 11 877, 27 44 785, 29 43 911, 30 04 829, 30 38 102, 30 38 141, 30 38 692, 31 20 884, 32 05 737, 32 09 673, 32 14 836, 32 38 396, 33 20 975, 33 26 440, 33 27 328, 33 27 329, 34 20 236, 34 27 498, 35 19 714, 35 29 827, 36 05 199, 36 05 885 and 36 38 409.
In connection with electronic switchgear, which can be connected via an external conductor with one terminal of a supply voltage source and only via another external conductor with a connection of a consumer, the provision of the supply voltage or supply current for the presence indicator and for the switching amplifier is not without problems, because the supply voltage or the supply current must be provided in the conducting state as well as in the inhibited state of the switchgear.
It is of no consequence whether the provision of a supply voltage or the provision of a supply current is addressed. Here, provision represents derivation from the voltage drop occurring at the switchgear, or from the operating current conducted via the switchgear (conducting state), or from the operating voltage present at the switchgear or from the residual current flowing across the switchgear (inhibited state). Therefore it is of no consequence whether the provision of a supply voltage or a supply current is addressed, because the oscillator and the switching amplifier of course require a supply voltage and a supply current.
Based on its operation as switchgear, practically no voltage drop should occur in the switchgear herein discussed in the conductive state and practically no residual current should flow in the inhibited state. However since, if no voltage drop is allowed to occur in the conductive state in switchgear with only two external conductors, no supply voltage for the oscillator and the switching amplifier could be obtained and, if no residual current is allowed to flow in the inhibited state, no supply current could be obtained, it is true for all electronic switchgear with only two external conductors that in the conductive state a voltage drop occurs and in the inhibited state a residual current flows.
It follows from what has been stated above that the voltage drop and the residual current should be as small as possible, even though in electronic switchgear with only two external conductors a voltage drop occurs in the conductive state and a residual current flows in the inhibited state in a way which is unintentional but necessary for the operation.
In the beginning it was stated that, among others, a switching amplifier, placed downstream from the oscillator, and an electronic switch are associated with the electronic switchgear on which the invention is based, and that the electronic switch can be controlled via the switching amplifier by the oscillator. The term switching amplifier is to be understood in a general way and encompasses the entire circuit between the signal output of the oscillator and the control input of the electronic switch, thus the entire signal transmission path between the oscillator and the electronic switch. In the electronic switchgear on which the invention is based (see, for example, German Patent No. DE-PS 30 04 829) the oscillator can be externally affected, i.e. damped by a metal part and the electronic switch is controlled depending on whether the oscillator oscillates or not. In practice the evaluation of the oscillating behavior of the oscillator is performed by a demodulator (first operating unit of the switching amplifier) and by a Schmitt trigger (second operating unit of the switching amplifier). The demodulator changes the oscillator voltage, i.e. a signal A.C. voltage, to an analog signal D.C. voltage and the Schmitt trigger changes a signal D.C. voltage which is analogous to the oscillator voltage to a digital output signal at its output with which, if required via a further operating unit of the switching amplifier or via a plurality of further operating units of the switching amplifier, the electronic switch is controlled. If the signal D.C. voltage at the signal input of the Schmitt trigger is above a comparison voltage applied to a comparison input, the output signal logically is, for example, 1 , if the signal voltage lies below the comparison voltage, the output signal logically is 0.
Oscillators require a certain run-up time in order to pass from the non-oscillating state to the oscillating state. This run-up time directly determines the maximum switching frequency of an electronic switchgear having an oscillator which can be affected in the manner described. The minimum time intervall between two actuation events of the oscillator must not be smaller than the run-up time of the oscillator; if the time intervall between two actuation events of the oscillator is smaller than the run-up time of the oscillator, the oscillator remains in the nonoscillating state.
Electronic switchgear of the type on which the invention is based is also used for counting tasks, so that the maximum switching frequency of such an electronic switchgear is of considerable importance.